In the field of electronic amplification it is often necessary to optimise efficiency, fidelity, device size and cost. For example radio frequency amplifiers are often implemented in mobile devices, requiring them to be extremely efficient to improve battery life, high fidelity to provide good quality signals, very small (preferably implemented all on one integrated circuit chip) to reduce the size and weight of the device, and low-cost (again a fully integrated solution is useful here).
One simple type of amplifier is the class-B push-pull stage amplifier. In amplifiers of this type complimentary or quasi-complementary devices are each used for amplifying opposite halves (positive/negative) of the input signal. The outputs of the devices are combined to yield an overall output. This is an efficient arrangement, however there can be a small mismatch in the cross-over region at the “joins” between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improved version is the class-AB amplifier. In class-AB operation the devices are biased so they are not completely off when not in use. Each device operates the same way as in Class B over half the waveform, but also conducts a small amount over the other half. As a result, the region where both devices simultaneously are nearly off is reduced. The result is greatly reduced crossover distortion. The specific implementation of Class-AB for high frequency operation (RF) uses active devices with tuned load (L-C tank) instead of complementary devices for improved head-room and frequency selectivity.
Amplifiers are often made to be differential in order to improve fidelity. Differential amplifiers typically work by employing two electronic amplifiers, measuring the difference between their inputs and multiplying this difference by a constant factor. This provides the advantage of effectively cancelling out voltages which are common to the outputs of amplifiers, producing an output signal having reduced harmonics level and dc offset voltages. An additional benefit of using a differential configuration, especially in high frequency implementations, is that available voltage swing is doubled compared to the corresponding single-ended implementation. This makes it possible to achieve larger output powers for a given impedance load, or to achieve a given output power on a larger load impedance.
Fully integrated differential class-AB amplifiers are known. These do however have drawbacks. For implementations where large current handling is required the tracks on any circuit board that carries the current must be very large to prevent electromigration and overheating (which if allowed would be both inefficient and dangerous). In radio frequency implementations, transformers called baluns are often used for coupling to the load. Large tracks mean that a high coupling coefficient cannot be obtained for magnetic coupling to the load and any balun that is used at the output cannot then be readily optimised. Therefore the combined load and balun structure typical of radio frequency class-AB differential power amplifiers is not ideal as it prevents simultaneous optimisation of quality factor and magnetic coupling.
An improvement of the class-AB amplifier is the class-E power amplifier. This uses resonant load at the working frequency for switching mode operation, and resonant load coupling for removing power dissipated on the harmonics. Due to their relatively simplistic design and good high-frequency performance, class-E amplifiers are regularly employed to amplify constant-envelope waveforms, where the transmitted carrier power is constant. A typical class E amplifier known in the art is shown in FIG. 1.
In the amplifier shown in FIG. 1, current passes through inductor L3 and through transistor MØ which operates as a switch driven by the signal to be amplified at V1. Capacitor C2 is placed in parallel with transistor MØ. C2 resonates the load at the operating frequency. Capacitor C1 and inductor L5 are placed in series together resulting in a tuned series LC circuit. This circuit is in series with another LC circuit formed by L4 and C5, having a reactive component jL4C5 and load impedance RL4C5.
As transistor MØ is periodically switched on and off by the input signal, the C1-L5 filter is tuned to the first harmonic of the input frequency and only allows through a sinusoidal current to load RØ.
L4 and C5 are the impedance matching circuit, so the load impedance RØ is transformed (step-up or step-down) to the desired level for the amplifier, or internal impedance, Rint. The maximum output power (Pout,max) is:
                    Pout        ,                  max          =                                                    Veff                2                                            R                ⁢                int                                      =                                          R                ⁢                int                            *                              Ieff                2                                                                        (        1        )            Where Ieff is the effective current and Veff is the available voltage for single-ended amplifiers with inductive load:
                    Veff        =                              (                                          Vdd                -                Vd                            ,              sat                        )                                2                                              (        2        )            Where Vd,sat is the saturation voltage of the active device MØ. For a given load impedance and supply voltage, impedance transformation is needed to achieve the desired output power:
                              R          ⁢                                          ⁢          ϕ          *          Rsource                =                              L            ⁢                                                  ⁢            4                                C            ⁢                                                  ⁢            5                                              (        3        )                                          F          rf                =                  1                      2            *            π            *                                          L                ⁢                                                                  ⁢                4                *                C                ⁢                                                                  ⁢                5                                                                        (        4        )            Where Frf is the working frequency of the amplifier.
The pair of resonant LC circuits creates a damped oscillation across the load of the amplifier. By using particular values for the components of the resonant LC circuits, a frequency of the damped oscillation can be chosen that ensures that the voltage across the transistor is low when the current is high, and the voltage is high across the transistor when the current is low. As power efficiency is usually determined according to the energy wasted in the process of amplifying a signal, the efficiency of an amplifier can be improved if the power across the transistor is minimised.
Differential class-E amplifiers are known, having the advantages of the good high-frequency performance of a simple class-E amplifier, together with those of a differential amplifier: common-mode and harmonics rejection, and increased voltage swing for a given supply
FIG. 2 shows an example of a differential class-E amplifier as derived from the single-ended structure of FIG. 1, with no optimisation excepting differential resonance of the load L3+L9 by C2. That is, in FIG. 2, two class-E amplifiers of the type shown in FIG. 1 are combined such that their respective outputs are connected.
In FIG. 2 the left-hand amplifier is largely the same as the class-E amplifier of FIG. 1. In the right-hand amplifier: inductor L9 corresponds to inductor L3, input V3 corresponds to input V1, transistor M1 corresponds to transistor MØ, capacitor C3 corresponds to capacitor C1, inductor L8 corresponds to inductor L5, inductor L10 corresponds to inductor L4, and capacitor C4 corresponds to capacitor C5.
The outputs of both amplifiers are connected by inductor L6 which is coupled to the load RØ by inductor L7, forming the output transformer.
This approach needs four inductor components for the harmonic tuning and output matching. As well as these four inductor components, a symmetrical inductance for the power amplifier load, and a transformer/balun for single-ended to differential transformation are also required. The large number of inductors required for this approach makes it undesirable for use in an integrated circuit (IC), as the large number of components will require a large amount of silicon area. Furthermore, it is difficult to pre-estimate inductive couplings between individual inductances. These “crossed” magnetic couplings shift the desired resonance frequency and degrade the impedance transformation.
What is needed is a differential class-E power amplifier with reduced crossed magnetic coupling between inductances and improved quality factor and magnetic coupling to the load, preferably small enough to be integrated on a one-chip implementation.